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William Stallings
Computer Organizationand Architecture
Chapter 5
External Memory
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Magnetic Disk
Metal or plastic disk coated with magnetizablematerial (iron oxiderust)
Range of packagingFloppy
Winchester hard disk
Removable hard disk
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Data Organization and
Formatting
Concentric rings or tracks
Gaps between tracks
Reduce gap to increase capacity
Same number of bits per track (variable packingdensity)
Constant angular velocity
Tracks divided into sectorsMinimum block size is one sector
May have more than one sector per block
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Disk Data Layout
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Fixed/Movable Head Disk
Fixed head
One read write head per track
Heads mounted on fixed ridged armMovable head
One read write head per side
Mounted on a movable arm
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Fixed and Movable Heads
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Removable or Not
Removable disk
Can be removed from drive and replaced withanother disk
Provides unlimited storage capacity
Easy data transfer between systems
Nonremovable disk
Permanently mounted in the drive
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Floppy Disk
8, 5.25, 3.5
Small capacity
Up to 1.44Mbyte (2.88M never popular)Slow
Universal
Cheap
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Winchester Hard Disk (1)
Developed by IBM in Winchester (USA)
Sealed unit
One or more platters (disks)Heads fly on boundary layer of air as disk spins
Very small head to disk gap
Getting more robust
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Winchester Hard Disk (2)
Universal
Cheap
Fastest external storageGetting larger all the time
Multiple Gigabyte now usual
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Removable Hard Disk
ZIP
Cheap
Very common
Only 100MJAZ
Not cheap
1G
L-120 (a: drive)
Also reads 3.5 floppy
Becoming more popular?
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Finding Sectors
Must be able to identify start of track and sector
Format disk
Additional information not available to userMarks tracks and sectors
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Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap3
TrackSync
ByteHead Sector CRC Sync
ByteData CRC
ST506 format (old!)
Foreground reading
Find others
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Characteristics
Fixed (rare) or movable head
Removable or fixed
Single or double (usually) sidedSingle or multiple platter
Head mechanism
Contact (Floppy)Fixed gap
Flying (Winchester)
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Multiple Platter
One head per side
Heads are joined and aligned
Aligned tracks on each platter form cylindersData is striped by cylinder
reduces head movement
Increases speed (transfer rate)
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Speed
Seek time
Moving head to correct track
(Rotational) latencyWaiting for data to rotate under head
Access time = Seek + Latency
Transfer rate
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RAID
Redundant Array of Independent Disks
Redundant Array of Inexpensive Disks
6 levels in common useNot a hierarchy
Set of physical disks viewed as single logicaldrive by O/S
Data distributed across physical drives
Can use redundant capacity to store parityinformation
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RAID 0
No redundancy
Data striped across all disks
Round Robin stripingIncrease speed
Multiple data requests probably not on same disk
Disks seek in parallel
A set of data is likely to be striped across multipledisks
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RAID 1
Mirrored Disks
Data is striped across disks
2 copies of each stripe on separate disks
Read from either
Write to both
Recovery is simple
Swap faulty disk & re-mirrorNo down time
Expensive
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RAID 2
Disks are synchronized
Very small stripes
Often single byte/word
Error correction calculated across correspondingbits on disks
Multiple parity disks store Hamming code errorcorrection in corresponding positions
Lots of redundancy
Expensive
Not used
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RAID 3
Similar to RAID 2
Only one redundant disk, no matter how large
the arraySimple parity bit for each set of corresponding
bits
Data on failed drive can be reconstructed from
surviving data and parity info
Very high transfer rates
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RAID 4
Each disk operates independently
Good for high I/O request rate
Large stripesBit by bit parity calculated across stripes on
each disk
Parity stored on parity disk
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RAID 5
Like RAID 4
Parity striped across all disks
Round robin allocation for parity stripeAvoids RAID 4 bottleneck at parity disk
Commonly used in network servers
N.B. DOES NOT MEAN 5 DISKS!!!!!
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Optical Storage CD-ROM
Originally for audio
650Mbytes giving over 70 minutes audio
Polycarbonate coated with highly reflective coat,usually aluminum
Data stored as pits
Read by reflecting laser
Constant packing density
Constant linear velocity
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CD-ROM Drive Speeds
Audio is single speed
Constant linier velocity
1.2 ms-1
Track (spiral) is 5.27km long
Gives 4391 seconds = 73.2 minutes
Other speeds are quoted as multiples
e.g. 24x
The quoted figure is the maximum the drive canachieve
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Comparison of Disk Layouts
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00 00FF
x 10 Min
Sec
Sector
Mode
DataLayered
ECC
12 byte
Sync
4 byte
Id2048 byte 288 byte
2352 byte
CD-ROM Format
Mode 0=blank data field
Mode 1=2048 byte data+error correction
Mode 2=2336 byte data
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Random Access on
CD-ROM
Difficult
Move head to rough position
Set correct speedRead address
Adjust to required location
(Yawn!)
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CD-ROM for & against
Large capacity (?)
Easy to mass produce
RemovableRobust
Expensive for small runsSlow
Read only
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Other Optical Storage
CD-Writable
WORM
Now affordable
Compatible with CD-ROM drives
CD-RW
Erasable
Getting cheaperMostly CD-ROM drive compatible
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DVD - whats in a name?
Digital Video Disk
Used to indicate a player for movies
Only plays video disks
Digital Versatile Disk
Used to indicate a computer drive
Will read computer disks and play video disks
Dogs Veritable DinnerOfficially - nothing!!!
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DVD - technology
Multi-layer
Very high capacity (4.7G per layer)
Full length movie on single diskUsing MPEG compression
Finally standardized (honest!)
Movies carry regional coding
Players only play correct region films
Can be fixed
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DVD - Writable
Loads of trouble with standards
First generation DVD drives may not read first
generation DVD-W disksFirst generation DVD drives may not read CD-
RW disks
Wait for it to settle down before buying!
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Foreground Reading
Check out optical disk storage options
Check out Mini Disk
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Magnetic Tape
Serial access
Slow
Very cheapBackup and archive
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Digital Audio Tape (DAT)
Uses rotating head (like video)
High capacity on small tape
4Gbyte uncompressed
8Gbyte compressed
Backup of PC/network servers